1. Field of the Invention
This invention relates to particulate solids and, more particularly but not exclusively, is concerned with satin white.
2. Description of the Prior Art
Satin white is a calcium sulphoaluminate and can be represented by the formula 3CaO.Al.sub.2 O.sub.3.3CaSO.sub.4.31H.sub.2 O. It is used inter alia in paper coating compositions as a white pigment which imparts high brightness and gloss and good printability to the coated paper. Satin white may be prepared by reacting aluminium sulphate with slaked lime, the resulting material being stored and transported in the form of a moist paste. Thus, in one process for preparing satin white a concentrated aluminium sulphate solution is added to a paste of slaked lime and the materials mixed in a high density mixer. The amount of water used in this method is such that the product is obtained in the form of a moist paste containing about 30% by weight of solids. In another process, a solution containing about 10% by weight of aluminium sulphate is sprayed into a slurry containing about 5% by weight of calcium hydroxide until reaction is completed. The resulting slurry is then pressure-filtered to give a moist filter cake, or paste, containing from 20 to 35% by weight of solids. On account of the high viscosity of aqueous suspensions of satin white it has not been found to be possible by conventional methods to prepare a paste containing more than about 35% by weight of solids and this means that when satin white is transported in this form it has associated with it nearly twice its weight of water.
Satin white contains a large amount of water of crystallisation and it rapidly becomes dehydrated if it is contacted with hot dry air. It can be dried by contacting the paste with air under closely controlled conditions of temperature and humidity, as is described in British Patent Specification No. 1,274,850, but careful control of the process is needed and, as satin white is generally required to be redispersed in water for incorporation into a paper coating composition, many customers prefer to receive satin white which is already dispersed in water.
In British Patent Specification No. 1,250,514 there is disclosed a process for dewatering satin white by means of a tube pressure filter whereby there can be obtained a filter cake, having a solids content ranging from about 50 to about 80% by weight, which is hard, brittle and non-sticky. However, although the filter cake is redispersable in water, apparatus must be provided by the customer for this purpose and a considerable amount of energy must be expended in effecting the redispersion. It is clearly desirable therefore to be able to produce a satin white slurry which has a solids content in excess of about 50% by weight and which can be dispersed in water without the need for expensive, high-powered equipment or for the expenditure of large quantities of energy.
It is known to increase the solids content of an aqueous solids suspension, to which there may be added a flocculating agent, by at least partially dewatering the solids suspension and adding a deflocculating agent thereto to form a slurry having an increased solids content.